Integrated gasification combined cycle (IGCC) power generating plants have demonstrable lower costs, improved reliability and improved efficiencies. The IGCC process relies on two-stage combustion with clean-up between the stages. The first stage includes a gasifier for partial oxidation of fossil fuel, i.e., coal, heavy fuel oils or the like, whereas the second stage utilizes a gas turbine combustor for burning the fuel gas produced by the gasifier to complete the combustion process. For example, it has been demonstrated that inherent fuel processing losses associated with fossil fuel gasification, in conjunction with the combined cycle, can deliver superior cycle efficiency. In a simple combined cycle power generating system, there is provided a gas turbine, one or more steam turbines, one or more generators and a heat recovery steam generator. The gas turbine and steam turbine may be coupled to a single generator in a tandem arrangement or multi-shaft combined cycle systems may be provided having one or more gas turbines, generators and HRSGs for supplying steam through a common header to a separate steam turbine generator unit. In the combined cycle, heat from the gas turbine exhaust is provided in heat exchange relation with a working fluid in the heat recovery steam generator for powering the steam turbines and, hence, generating electricity or mechanical work.
In recent years, there have been substantial improvements in thermodynamic cycles employing multi-component working fluids and a combination of absorption, condensation, evaporation and recuperative heat exchange operations to reduce irreversible losses typical of conventional Rankine cycles. Generally, these improved thermodynamic cycles are known as Kalina cycles and afford demonstrable and substantial improvements in thermodynamic cycle efficiency. Kalina cycles use two interactive subsystems. The first subsystem involves a heat acquisition process for a multi-component working fluid comprising, for example, preheating, evaporating, superheating, regenerative feed heating and power generation. The second subsystem consists of a distillation/condensation subsystem (DCSS). The efficiency improvements of the Kalina cycle over the Rankine cycle are a result of the use of a multi-component working fluid, preferably an ammonia/water mixture, the components of which have different boiling points at the same pressure. The compositions of the vapor and liquid streams change at different points throughout the cycle and the sub-systems enable closer matching of the enthalpy-temperature characteristics of the working fluid and the heat source used to evaporate the working fluid and the heat sink used to condense it.
In the heat acquisition subsystem, the Kalina system closes the mismatch between the enthalpy-temperature characteristics of the heat source and working fluid as the working fluid passes through the boiler. These energy losses, typical of the Rankine cycle, are reduced by taking advantage of the changing temperature-enthalpy characteristics of the multi-component working fluid as it evaporates.
In the second subsystem, i.e., the DCSS of the Kalina cycle, the spent working fluid after expansion through the turbine, is too low in pressure and too high in ammonia concentration to be directly condensed at the temperature of available coolant. The working fluid therefore can only be partially condensed and a lean solution is mixed with a two-phase precondensed flow from a recuperative heat exchanger, thereby forming a lower concentration of ammonia/water mixture which can be fully condensed at available coolant temperature. The lean condensate is subsequently distilled recuperatively against the turbine exhaust to regenerate the working composition for the heat acquisition subsystem. The Kalina cycle has been the subject of a number of patents including U.S. Pat. Nos. 4,586,340; 4,604,867; 5,095,708 and 4,732,005, the disclosures of which are incorporated by reference. The continued quest for increased efficiencies in power generation equipment has resulting in combining the Kalina bottoming cycles in an integrated gasification combined cycle power generating system in accordance with the present invention.